1. Field
Disclosed herein is a process for the synthesis of diene copolymers comprising at least two blocks, one of the blocks being composed of a diene elastomer. This process of synthesis comprises the coupling reaction of an azide-functional polymer with a functional diene elastomer. The present disclosure also relates to such diene copolymers and to the rubber compositions comprising them, in particular for the purpose of an application in vehicle tyres.
2. Description of Related Art
The synthesis of block copolymers is not always simple to control, in particular when one of the monomers may be involved in secondary reactions. The synthesis of block diene copolymers is not free from this difficulty. Various methods of synthesis are generally employed to prepare such block copolymers.
Thus, according to one method of synthesis, the two monomers are successively polymerized by anionic polymerization. This is one of the most well known methods to a person skilled in the art, which consists in polymerizing the diene monomer by the anionic route and then using the elastomer (living anionic chain) thus obtained as macroinitiator for the polymerization of the second monomer, still by the anionic route.
Certain difficulties may occur, depending on the nature of the second monomer. Thus, during the second stage of anionic polymerization, side reactions brought about by the presence of certain functional groups on the second monomer may compete with the polymerization. By way of example, if an ester functional group is present on the second monomer, the polymerization can be accompanied by an addition-elimination reaction which halts the growth of the chains of the second block and modifies the chemical structure of the final product and thus consequently its properties.
Moreover, the anionic polymerization of some monomers is highly exothermic and the kinetics of polymerization are very rapid (of the order of a minute). Many studies have been published relating to additives (LiCl or ROLi, for example) which make it possible to reduce the reactivity of the chain end by complexing and thus to reduce the proportion of secondary reactions, with greater or less success. It is thus not easy to control the anionic polymerization of such monomers.
Another method, developed by Stadler et al. (Macromolecules, 1995, 28, 3080-3097; Macromolecules, 1995, 28, 4558) and Teyssie and Elf Atochem (Patents EP 0 749 987 B1 and EP 0 524 054 B1), is the use of diphenylethylene (DPE) at the end of the polymerization of the diene monomer and the decrease in the reaction temperature (−40° C., for example), making it possible to obtain a lithiated macroinitiator which is much less reactive during the initiation of the second (meth)acrylic monomer. Despite a reactivity reduced by the use of DPE and/or of a lithium salt (LiCl or ROLi, for example) and of a low temperature, it is often difficult to completely prevent the side reactions and the control of the anionic polymerization of the second monomer can remain complicated.
Another method of synthesis of block diene copolymers is the combination of an anionic polymerization and of atom transfer radical polymerization (ATRP). The synthesis of a first diene block by anionic polymerization can be followed by a termination reaction which makes it possible to obtain a polymer functionalized at the chain end by a halogenated group; this halogen atom would make possible the initiation of the ATRP of the second monomer. The functionalization of the anionic chain end by a halogen atom, for example bromine, can be carried out in two stages: (a) the reaction of the living lithiated anionic chain with an epoxide, with the aim of replacing the carbanion by a lithiated oxanion which is less reactive with regard to nucleophilic substitutions, (b) the oxanion will, in a second step, react with the halide. Only the halide reacts by virtue of the decrease in reactivity of the anionic chain end. The polymer functionalized at the chain end by a halogenated group might then act as macroinitiator for the ATRP. This synthetic route was used by Matyjaszewski et al. (Macromol. Chem. Phys., 1999, 200, 1094-1100) for the synthesis of PS-b-PMMA block copolymer.
Nevertheless, this method proves to exhibit numerous difficulties depending on the monomers used. This is because, when the halogenated compound used to functionalize the diene elastomer resulting from the anionic polymerization exhibits two halogenated sites, as is the case for example, with 2-bromo-2-methylpropanoyl bromide, the functionalization can also result in coupling of the living anionic chain by reaction of two living diene elastomer chains with the bihalogenated compound. In addition, a major obstacle to this method of synthesis originates from the presence of pendant double bonds in the diene polymer. This is because, during the ATRP of the second monomer, radical addition reactions on the double bonds of the diene elastomer take place, resulting in the change in the macrostructure, indeed even in crosslinking, resulting in the formation of a gel. Good control of the radical polymerization is thus impossible.
These disadvantages have in particular been demonstrated by the Applicant Companies during various tests employing butadiene and methacrylate as monomers.